Multi-carrier code division multiple access (MC-CDMA), multiple-input multiple-output (MIMO) and beamforming technologies are proposed as candidates for next generation broadband wireless communication systems. For example, MC-CDMA, MIMO and beamforming techniques have been considered key technologies for the IEEE 802.16m communication standard to be used in next generation WiMAX™ systems.
Beamforming signals formatted in accordance with MC-CDMA techniques is challenging. Channel information between the transmit antennas and the intended receiver antenna(s) needs to be known in order to perform beamforming effectively. The channel information may be frequency dependent. When beamforming, the same signal is transmitted from multiple transmit antennas with different magnitudes and phases so that the signals from multiple transmit antennas are optimally combined at the intended receiver antenna(s). The magnitudes and phases are determined based on the channel information.
The channel information between the transmit antennas of a first device and the receiver antenna(s) of a second device can be obtained from signals transmitted by the second device to the first device when the property of the channel symmetry is utilized. For example, a wireless base station can derive the downlink channel information based on uplink signals sent from a wireless mobile station.
For a base station to compute channel information for each wireless mobile station, the uplink signals from different mobile stations need to be separated in frequency and time. This is generally not the case with MC-CDMA communication techniques.
When using MC-CDMA techniques, the received uplink signals from one mobile station overlaps (in frequency and time) with the uplink signals of other mobile stations. MC-CDMA techniques, by design, spread and overlap signals of all mobile stations over a frequency spectrum. For example, in a WiMAX system, when a mobile station initially enters a network, it sends a wide band uplink ranging signal (i.e., a CDMA signal) in a frequency band shared by other mobile stations. The received signal at the base station is a combination of the uplink signals from multiple mobile stations.
Since MC-CDMA techniques spread the signal over a relatively wide frequency spectrum, there are not enough uplink symbols within the channel coherent bandwidth for a reliable channel information computation. Furthermore, in a multi-path environment, the channel information is frequency dependent, which makes computing the channel information even more complex.
Convention beamforming schemes cannot be used for MC-CDMA signal formatting techniques. A new scheme is needed for computing beamforming weight vectors based on channel information derived from signals transmitted according to MC-CDMA techniques.